Solar heat is a type of free and renewable energy for heating houses in spring, fall and winter seasons, and providing hot water in summer. Solar heat/energy is used to be collected by a large size collector, from which the collected heat is transfer into a house by blowing air for heating it, or stored in a water tank for providing hot water. Actually, this is a direct way to collect solar energy, and it has many weaknesses. First, the solar heat with direct collection cannot be a stable heat source. It can only be collected on sunny days, no heat comes out on those cloudy days. Second, the direct solar heat is only for heating rather than cooling a house. Third, solar heat collecting systems have a large size/area collector with some pipe, fan or tank, which cost and installing fee are not inexpensive. Also, the systems for heating house and providing hot water are used to be designed as different systems, which cannot share the collector.
In fact, there is an indirect way to collect solar heat and it is much more stable for both heating and/or cooling a house in spring, summer, fall and winter in mild/hot climate regions. It is well known that the outdoor air has a temperature difference between daytime and nighttime due to the sun heats up the atmosphere and surface of earth in daytime and the stored heat dissipates into space in the nighttime. The daily temperature difference can be from 5° C. to 15° C. in all seasons and most geographical locations. Usually, a daily high temperature peak occurs between 2:00 pm and 7:00 pm, while a daily low temperature peak takes place between 3:00 am and 8:00 am.
Heat pumps are used in applications of space heating or cooling for human comfort in offices and residential homes. Heat pumps are able to move heat energy between one environment and another. When a heat pump is installed in a house, it can bring heat into (heating) or take heat out of (cooling) the house.
In heating mode of a heat pump for example, unlike an electric or gas heater, a heat pump moves heat from an outdoor space into an indoor space rather than creating it. Coefficient of Performance (COP) is used to present how efficient a heat pump works. COP is defined as the ratio of useful heat output to electricity energy input. Heating a house on a mild day at 10° C., a typical air source heat pump has a COP of 3 to 4, while an electric heater with pure resistance has the COP of 1. COP is also applied in cooling mode. The theoretical COP for a heat pump for both heating and cooling modes can be expressed as:
                              COP          heating                =                                            Δ              ⁢                                                          ⁢                              Q                hot                                                    Δ              ⁢                                                          ⁢              A                                =                                    T              indoor                                                      T                indoor                            -                              T                outdoor                                                                        (        1        )                                          COP          cooling                =                                            Δ              ⁢                                                          ⁢                              Q                cool                                                    Δ              ⁢                                                          ⁢              A                                =                                    T              indoor                                                      T                outdoor                            -                              T                indoor                                                                        (        2        )            
Where ΔQhot is the amount of heat moved from outdoor coil to indoor coil, ΔQcool is the amount of heat moved from indoor coil to outdoor coil. ΔA is the electrical energy taken by compressor and fans. Tindoor and Toutdoor are the temperatures of the indoor and outdoor coils in degree Kelvin (K).
COP of a heat pump is various when the difference of outdoor temperature and indoor temperature changes. FIG. 1 gives a roughly curve 10 of COP versus the absolute value of temperature difference between indoor and outdoor coils |Tindoor−Toutdoor|, which is applied for both heating and cooling. When the difference between indoor and outdoor temperatures is higher, the heat pump needs to run more cycles for moving the same amount of heat, and COP is lower. When the difference between indoor and outdoor temperatures is lower, the same amount of heat can be moved less cycles, COP becomes higher.
According to the COP characteristics, an air source heat pump can be used to collect solar heat indirectly for both heating and cooling by using the daily temperature difference. For heating a house in spring, fall or winter, a heat pump should work as much as possible in the daily highest temperature time when the daily COP is the largest value. For cooling the house in summer, the heat pump should work much in the lowest daily temperature time for the best daily COP. In this way, the COP of the heat pump always works with the best daily COP for both heating and cooling.
Unfortunately, current air source heat pumps always work harder with the worst daily COP. FIG. 2A illustrates the outdoor temperature curve on a mild day and a heat pump working status in heating mode. The curve 12 expresses the daily temperature change, and boxes 14 on the time dimension show the heat pump working status with each starting and working time. At the low temperature peak happens around 6:00 am, the heat pump starts more frequently and works longer in each working time when the house loses more heat and heat pump works in its worst working condition with lowest daily COP. At the high temperature peak around 6:00 pm, the heat pump starts less than the average rate and works shorter in each working time when the house loses less heat and heat pump works in its best working condition.
In FIG. 2A, the average temperature of the best three hours is 15.5° C. from time 16:00 pm to 19:00 pm, then the average best COPideal=(273.15+35)/(35−15.5)=15.8, where Tindoor and Toutdoor are assumed as 35° C. (an indoor coil temperature under the blowing of an indoor fan) and 15.5° C. respectively. The average COP in the rest time can be calculated as 11.2 by using the method of differential equation. If the heat pump can work for three hours in high temperature duration to deliver heat for heating the house in the whole day, the theoretical COP can be increased for 15.8−11.3=4.6, then the increase rate is 4.6÷11.2=41%, which can be treated as the improvement in practical COP.
Due to the house needs the least heat in the duration of highest daily COP of a heat pump, a heat storage tank is the only condition to make a heat pump works in this duration. Thus, the heat pump can work with its best daily COP to store the heat into the tank for a whole day using, and the stored heat will be released from the tank into house in the rest time of the day.
FIG. 2B illustrates this type of time shifting mode. In FIG. 2B, the box 14 indicates the heat pump working for heating the house, the boxes 16 mean that the heat pump working for charging the heat into the heat pump and the boxes 18 express the heat stored tank discharging for heating the house. In the low temperature duration, the heat pump doesn't work, the house is heated by stored heat in the tank. In the high temperature duration, the heat pump works for two or more hours to heat the house and/or charge heat into the tank.
Such the system can be implemented by incorporating a heat storage tank into a heat pump system as shown in FIG. 3. Based on a typical air source heat pump system, a daily heat storage tank 24 is applied between the outdoor unit 20 of the heat pump and its indoor coil 28. The refrigerant tubes 22 connects the daily heat storage tank 24 and outdoor unit 20, while the water tubes 26 connects the daily heat storage tank 24 and the indoor coil 28. A fan/blower 30 is in charge of exchanging heat between the indoor coil 28 and indoor air. The heat storage tank 24 can be charged or discharged by either heat or cool, so the stored heat or cool can be a source to heat the house in spring, fall and winter or to cool the house in summer Some heat exchanger and water pump can be used to make this system works much efficient than a normal heat pump, but when the heat storage tank is used to store a certain amount heat for a whole day using, its feasibility does need to be discussed. Usually, water is a very good material to store heat or cool for countless times charging and discharging.
In a mild climate area, 80,000˜120,000 BTU daily heat is an essential amount for heating a house in spring, fall and winter. If water is used for storing 150,000 BTU heat to cover the worst case usage, and assume that the water tank is charged and discharged by heat between 30˜45° C., that means 15° C. or 27° F. temperature difference, then the amount of water can be calculated as 150,000 BTU/27° F.=5556 lbs or 666 gallons, since the daily heat storage tank is installed in an indoor space, such the amount of water makes the tank very big. In cooling operation, the same amount of water can be charged or discharged by cool between 5˜20° C., which is also 15° C. or 27° F. temperature difference, then the same amount of cool is used for cooling the house for one day. It is obvious that the system in FIG. 3 has a big size tank when using water as the heat storing material.
The system in FIG. 3 charges the heat storage tank 24 by a heat pump, discharges the tank 24 by a water pump. This means that the temperature of the water tank 24 must at least 5° C. higher than the room temperature for heating operation, or at least 5° C. lower than the room temperature for cooling operation. Thus, the temperature difference for storing heat/cool is about 10˜15° C. If the water tank can be charged or discharged by a heat pump, then the temperature difference can be 45° C.−5° C.=40° C. for both heating and cooling. The invention is just created by using heat pump charging/discharging strategy to implement a heating and cooling system with a compact tank and high efficiency. Also, the invention system also provides water pump working modes to charge or discharge the heat storage tank partially to increase the efficiency further.
In residential house applications, the most efficient heat pump is the ground source heat pump or geothermal heat pump. It uses an extra water loop to exchange heat between ground and the outdoor coil. The heating COP of a geothermal heat pump can be 4 to 5 because the temperature of ground is a constant temperature about 10° C. to 15° C. in all seasons, so the annual electricity bill can be reduced for 30˜40% by using a geothermal heat pump for both heating and cooling. However, the initial cost is nearly twice the price of a regular heat pump, furthermore the cost is much higher for drilling ground and installing water pipe. The total cost is around $25,000 in average for installing a geothermal heat pump that means the investment will be gotten back in a longer term such as 10 years or more.
Compared with geothermal heat pump, the invention has a much lower installation cost with significantly improved COP in spring, summer, fall and whole or a part of winter in south regions. Therefore, the invention is suitable in use in warm and hot climate regions, while a geothermal heat pump is a good solution for cold climate regions, where the winter is longer with a high annual heating cost.